I'm not 100% sure of the answer, b/c i've never actually done this. But, the ammonia spike you would normally see is caused by the die off. and in your case, you should not see a die off since the rock is already dead. You really don't need to seed it either. Drop it in, and if you wanna get the ball rolling, go to the grocery store and drop in a piece or two of raw, uncooked shrimp. That will get the ammonia to spike, etc. If your tank is up and running, i don't see why it would be a problem to just throw it in. I would, however, throw it in a bucket with a few powerheads to get the spiders out. someone else might have a better answer b/c, like i said, i've never actually done this. but, that was just my speculation.